This application is based upon and claims the benefit of priority of the prior Japanese Patent Application 2003-331539 filed on Sep. 24, 2003.
The present invention relates to a spark plug for a gas engine which is equipped with a ground electrode body and a center electrode body.
The Japanese Patent Application Laid-open No. 2002-93547 (corresponding to the U.S. patent application Publication 2002/0003389 A1) discloses a conventional spark plug for a gas engine that includes a ground electrode body and a center electrode body. A ground electrode of the ground electrode body is made of a material containing iridium as a main component. The ground electrode is welded on a surface of a ground electrode base material opposing to the center electrode body. The ground electrode protrudes toward the center electrode body. The ground electrode base material is made of a heat-resistant nickel alloy. The ground electrode is impacted in a recess of the ground electrode base material.
According to this conventional spark plug for a gas engine, there is no evaluation with respect to the relationship between the thickness of the ground electrode base material in an intrusion direction of the ground electrode (hereinafter, simply referred to as the thickness of the ground electrode base material) and the intrusion length of the ground electrode that represents the length of a portion of the ground electrode intruded into the ground electrode base material in the intrusion direction (hereinafter, simply referred to as the intrusion length of the ground electrode).
Furthermore, the Japanese Patent Application Laid-open No. 2002-313523 discloses another conventional spark plug for a gas engine that includes a ground electrode of the ground electrode body and a center electrode of the center electrode body which are made of a material containing iridium or platinum as a main component. This conventional spark plug satisfies a condition D1/D2>0.9 when the diameter of the ground electrode is D1 and the diameter of the center electrode is D2.
In general, the spark plug for a gas engine is subjected to high-temperature environments. The above-described conventional spark plugs are dissatisfactory in the durability when they are subjected to the high-temperature environments as described below.
For example, the spark plug for a gas engine is disposed in a sub combustion chamber of an indirect lean burn gas engine. In this case, the gas mixture having a high concentration is introduced into the sub combustion chamber and accordingly the temperature of the sub combustion chamber becomes very high. Therefore, when the spark plug is used in the indirect lean burn gas engine, the spark plug is forcibly subjected to very severe high-temperature environments (e.g., 800° C. or above).
When no evaluation is given with respect to the relationship between the thickness of the ground electrode base material and the intrusion length of the ground electrode, there is a possibility that the durability of the ground electrode body is dissatisfactory.
It is now assumed that the intrusion length of the ground electrode is excessively short relative to the thickness of the ground electrode base material. In other words, when the depth of the ground electrode intruded into the ground electrode base material is very shallow, the strength for holding the ground electrode with the ground electrode base material is insufficient. The ground electrode tends to fall off the ground electrode base material. The durability of the ground electrode body is not good.
Furthermore, to increase the strength for holding the ground electrode with the ground electrode base material, it may be possible to increase the ratio of the intrusion length of the ground electrode to the thickness of the ground electrode base material. In other words, the depth of the ground electrode intruded into the ground electrode base material may be increased.
However, there is a significant difference in the thermal expansion coefficient between the ground electrode and the ground electrode base material. The ground electrode is made of a material containing iridium or platinum as a main component, while the ground electrode base material is made of a heat-resistant nickel alloy. The heat-resistant nickel alloy has a larger thermal expansion coefficient. As described above, when installed in a gas engine, the ground electrode body is forcibly subjected to the high-temperature environment. A significant amount of thermal expansion will occur between the ground electrode and the ground electrode base material which are welded together.
If the depth of the ground electrode intruded into the ground electrode base material is excessively deep, the ground electrode base material will cause an undesirable warpage or swell at a back surface which is opposite to the inner surface on which the ground electrode is intruded. An undesirable stress will act on the bonded portion of the ground electrode and the ground electrode base material which are welded together.
Accordingly, when the depth of the ground electrode intruded into the ground electrode base material is excessively deep, the ground electrode tends to fall off the ground electrode base material. The durability of the ground electrode body is dissatisfactory.
On the other hand, when the ratio D1/D2 of the diameter D1 of the ground electrode to the diameter D2 of the center electrode is set to be larger than 0.9, the ratio D1/D2 may have a small value and accordingly the ground electrode may have a very small diameter D1. In this case, the spark discharge tends to occur between the center electrode and the ground electrode base material. It is needless to say that the spark discharge should occur between the center electrode and the ground electrode. The ground electrode base material is not the member for causing the spark discharge. The ground electrode base material is the member supporting the ground electrode.
If the spark discharge occurs between the center electrode and the ground electrode base material, the ground electrode base material will be forcibly subjected to the high-temperature environment and will wear rapidly due to high-temperature oxidation. The required discharge voltage will increase when the spark plug is used for a long time. Such a spark plug for a gas engine will have insufficient durability.